Assessing angular and linear misfits in zirconia frameworks through torque/time graph interpretation: A proof of...

Passive fit in implant-supported complete-arch prostheses is the key to long-term success. The aim of this in vitro, proof-of-concept study was to examine whether torque/time graphs generated from a surgical motor during the prosthetic screw insertion process show consistent patterns of digitally controlled angular and linear misfits in monolithic zirconia frameworks.

Eight high-strength, low-translucency zirconia blocks, suitable for complete-arch prosthetic frameworks, were manufactured and assessed. A standardized edentulous model with three implants was used. The two outer prosthetic screws were screwed and secured, whereas the central screw was tightened using a surgical motor, allowing real-time acquisition of torque/time graphs for each framework. Framework design was intentionally modified to introduce controlled degrees of angular and linear misfit. The resulting torque/time curves were analyzed to identify graphical patterns associated with each specific level of angular and linear misfit.

Torque/time graphs were recorded for all zirconia frameworks. The control sample showed a gradual rise in torque to 13.7 N cm with a stable 5 revolutions per minute (rpm), indicating optimal passive fit. Frameworks with small linear (≤80 µm) or angular (≤1.9°) discrepancies displayed similar, clinically acceptable patterns. Linear misfits beyond 120 µm resulted in delayed torque progression and higher resistance, whereas at 160 µm, screw engagement failed, with torque plateau at 2.0 N cm. Angular misfits greater than 2.9° resulted in irregular torque curves and early mechanical tension. Deviations of 3.8° produced unstable torque, reduced rpm, and incomplete screw seating. At 4.8°, no engagement of the screw occurred, with torque values less than 2.0 N cm.

Within the limitations of this in vitro, proof-of-concept study, interpreting torque/time graphs obtained during prosthetic screw insertion may represent a promising non-invasive approach to detect misfits in zirconia complete-arch frameworks. This suggests a feasible approach to improve the objectivity of passive fit assessment in clinical practice, although further validation, both in vitro and in vivo, is recommended.